Some techniques for communicating in a network involve modulating data onto signals transmitted over a shared medium. For example, orthogonal frequency division multiplexing (OFDM), also known as Discrete Multi-Tone (DMT), is a spread spectrum signal modulation technique in which the available bandwidth of the medium is subdivided into a number of narrowband, low data rate channels or “carriers.” To obtain high spectral efficiency, the spectra of the carriers are overlapping and orthogonal to each other. Data are transmitted in the form of symbols that have a predetermined duration and encompass some number of carriers. The data transmitted on these carriers can be modulated in amplitude and/or phase, using modulation schemes such as Binary Phase Shift Key (BPSK), Quadrature Phase Shift Key (QPSK), or m-bit Quadrature Amplitude Modulation (m-QAM). Alternatively, other modulation techniques can also be used.
Many wireline and wireless network technologies use a Physical Protocol Data Unit (PPDU) format that includes preamble, header, and payload portions. The preamble typically consists of a predetermined signal (e.g., transmitted on one or more OFDM symbols) used for start of PPDU detection and initial channel estimation to decode the header. The header provides network management information for the recipient to properly decode the payload (e.g. tone map index). Additionally, the header may provide information for proper network operation (e.g. virtual carrier sense). The PPDU is usually followed by a subsequent short PPDU that provides the acknowledgement from a receiver of the PPDU. For noisy wireline and wireless media, the preamble, header, and acknowledgement are typically designed to operate under various channel conditions and thus tend to be relatively long in duration in order to be reliable. However, these items add overhead relative to the portion of the transmissions that carry the payload.